The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
A class of machines exist in the art generally known as “scroll machines” for the displacement of various types of fluids. Such apparatus may be configured as an expander, a displacement engine, a pump, a compressor, etc., and many features of the present teachings are applicable to any one of these machines. For purposes of illustration, however, the disclosed machines are in the form of a hermetic refrigerant compressor. Generally, a scroll machine comprises two spiral scroll wraps of similar configuration, each mounted on a separate end plate to define a scroll member.
The two scroll members are typically inter-fitted together with one of the scroll wraps being rotationally displaced 180° from the other. The machine operates by orbiting one scroll member (the “orbiting scroll”) with respect to the other scroll member (the “fixed scroll” or “non-orbiting scroll”) to make moving line contacts between the flanks of the respective spirals, defining isolated, crescent-shaped pockets of fluid moving from an inlet to an outlet.
The spirals are commonly formed as involutes of a circle, and ideally there is no relative rotation between the scroll members during operation; i.e., the motion is purely curvilinear translation (i.e., no rotation of any line in the body). The fluid pockets carry the fluid to be handled from a first zone in the scroll apparatus where a fluid inlet is provided, to a second zone in the apparatus where a fluid outlet is provided. The volume of a sealed pocket changes as it moves from the first zone to the second zone. At any one instant in time, there will be at least one pair of sealed pockets; and when there are several pairs of seal pockets at once, each pair will have different volumes. In a compressor, the second zone (or outlet) is at higher pressure than the first zone (or inlet) and is physically located centrally in the apparatus, the first zone being located at the outer periphery of the apparatus.
Two types of contacts define the fluid pockets defined between the scroll members: axially extending tangential line contacts between the spiral faces or flanks of the wraps caused by radial forces (“flank sealing”), and area contacts caused by axial forces between the plain edge surface (the “tips”) of each ramp and the opposite end plate (“tip sealing”). For higher efficiency, good sealing must be achieved for both types of contacts.
The concept of a scroll-type machine has been recognized as having distinct advantages. For example, scroll machines have high isentropic and volumetric efficiency, and, hence, are relatively small and lightweight for a given capacity. They are, typically, quieter and vibration-less than many compressors types because they do not use large reciprocating parts (e.g., pistons, connecting rods, etc.), and because all fluid flow is in one direction with simultaneous compression in plural opposed pockets, there are less pressure-created vibrations. Such machines also tend to have higher reliability and durability because of the relatively few moving parts utilized, the relatively low velocity of movement between the scrolls, and an inherent forgiveness to fluid contamination.
Scroll compressors should not be rotated in reverse, however, as the scrolls can become damaged. One way a scroll compressor may operate in reverse in when compressed refrigerant remaining in the discharge line returns to the compressor and cause the scrolls to run in reverse. This reverse rotation of the scrolls may damage compressor components, including the scrolls, as high-pressure fluid flows to the lower-pressure inlet side of the scrolls. Accordingly, a short discharge line minimizes the volume of refrigerant contained therein and, once the compressor has shut down, a minimal amount of gas will return to the compressor and cause the scrolls to run in reverse.
With an externally mounted muffler, a short discharge line is prone to break because the muffler's larger mass vibrates while the compressor is running. To correct this, the discharge tube for an externally mounted muffler may have generally a longer length of tubing to the compressor. The longer discharge tubing, however, increases the volume of refrigerant present in the discharge line and cause the scrolls to reverse orbit upon shut down.